1. Field of the Invention
This invention relates to a ferromagnetic iron alloy powder suitable for constituting the magnetic layer of a magnetic tape, magnetic disk or other magnetic recording medium.
2. Background Art
In recent years, the need has increased for magnetic recording media that offer not only improved recording capacity but also enhanced reliability and durability.
In order to boost recording capacity, an intense effort is underway to shorten the wavelength of the signals used so as to enable next-generation magnetic recording media to achieve still higher recording density. This in turn has created a need for finer ferromagnetic powders with superior properties. Unless the size of the acicular particles is much smaller than the length of the region recording signals on the short-wavelength side, a state of magnetization transition clearly cannot be produced, making recording substantially impossible. Acicular ferromagnetic iron alloy powders are therefore required to have sufficiently small major axis length. To be specific, such powders are desirably composed of very fine particles with a major axis length of not greater than 80 nm. Moreover, a ferromagnetic iron alloy powder used in a high-density-ready magnetic recording medium must have high coercive force Hc in order to maintain magnetization in, and secure an output from, the high-density medium.
Owing to advances in the system technologies, moreover, the conventional inductive head is being replaced by high-sensitivity MR and GMR heads. When a ferromagnetic iron alloy powder having a high saturated magnetization os intended for use with a conventional head is used with such a head, the high-sensitivity MR or GMR head is saturated. Unlike heretofore, therefore, a low σs powder is required. The powder may, for instance, be required to have a low σs of not higher than 130 emu/g, though the appropriate value differs depending on the system.
In light of these circumstances, ferromagnetic iron alloy powders for magnetic recording media have, from the functional aspect, come to require as concomitant properties: ultra-fine particles not exceeding 80 nm, high coercive force, and low σs of not greater than 130 emu/g.
Improved reliability is needed because the importance of avoiding loss of stored data increases with increasing magnetic recording medium capacity. Tape for storing data requires particularly high reliability and, because of this, improvement of tape storage stability is desirable. In other words, it is essential for the ferromagnetic iron alloy powder itself to have high weather resistance (oxidation resistance) so as to be capable of remaining stable, unaffected by the surrounding environment.
The point is, therefore, that a ferromagnetic iron alloy powder suitable for a high-density magnetic recording medium must simultaneously satisfy the requirements of fine particle size, high Hc, low σs, and good weather resistance (oxidization resistance). At present, however, no ferromagnetic iron alloy powder is available that simultaneously and adequately meets these criteria.
It is extremely difficult to simultaneously achieve high Hc and oxidization resistance in a ferromagnetic iron alloy powder of reduced particle size. In order to impart oxidization resistance, the metal particle surfaces must be covered with a uniform and dense oxide film. Such an oxide film is hard to form on ultra-fine particles. This can be seen, for example, from the fact that a method of oxidizing particle surfaces using a weak oxidizing gas (weak oxidizing gas obtained by mixing water vapor or oxygen gas with an inert gas), such as taught by JP 4-230004A, is effective when applied to ordinary particles of over 100 nm but does not work with ultra-fine particles having a major axis length of 80 nm or less because the shape of the internal metal core portion deforms to lower the coercive force.
An object of the present invention is therefore to overcome the foregoing problems by providing a ferromagnetic iron alloy powder suitable for a high-density magnetic recording medium that simultaneously meets the requirements of fine particle size, high Hc, low σs, and excellent weather resistance (oxidization resistance).